Induction heating is a method of melting or otherwise heating a quantity of metal, not by applying heat externally, but by using the metal workpiece as its own heat source. An induction melting furnace generally includes a container for holding metal to be melted, an induction coil surrounding the container, and a power supply having an output circuit connected across the coil. In operation, the power supply creates current flow through the coil which, in turn, causes an alternating magnetic field to pass through the metal within the container. This field induces current flow in the metal so that the metal is heated internally by resistance heating.
In its electrical characteristics, an induction furnace is often visualized as equivalent to a transformer with a primary coil and a melt charge which behaves like a shorted secondary coil. The power released into the melt charge is proportional to the square of the current in the induction coil (primary coil): EQU P=I.sub.melt.sup.2 R
where:
P=power; PA1 I.sub.melt =current in the melt bath; and PA1 R=resistance of the melt. PA1 n=number of coil turns; PA1 I.sub.coil =current in the coil; PA1 At t.sub.0 : One set of SCRs is fired. Positive current delivered to RLC, resulting in positive power dissipation in the load. PA1 At t.sub.1 : Sine-curve behavior of RLC causes inverter current to become zero and then negative (shaded area 101a). Because current is negative while voltage is still positive, power to RLC becomes negative (shaded area 111a). This represents power not dissipated by the load. Reversal of current through first set of SCRs causes them to shut off. PA1 At t.sub.2 : Alternate set of SCRs is fired, causing reversal of direction of voltage across the RLC. Because current and voltage are now both of the same polarity, power is again dissipated in the load. PA1 At t.sub.3 : Inverter current crosses zero point and becomes positive (shaded area 101b). Because current is positive and voltage is negative, no power is dissipated (shaded area 111b). PA1 At t.sub.4 : First set of SCRs is again fired. Current, voltage, and power are all positive and the cycle begins again. PA1 I--amplitude of inverter current; PA1 f--frequency of SCR firing (.sup.1 /T) ##EQU6## t--time interval in which energy is being returned to the DC source.
Further, the current induced within the melt charge is equal to the current in the primary coil times the number of turns in the coil, or: EQU I.sub.melt =nI.sub.coil
where:
therefore EQU P=n.sup.2 I.sub.coil.sup.2 R
Since melt charges are almost always of metals having low resistance, providing high power to the melt charge requires either a high number of turns or a high current in the induction coil. These, in turn, yield poor efficiencies. Induction coils usually have low power factors.
To offset high inductance of the coil, it is usual to include a capacitor in the circuit, creating an RLC oscillating circuit. As is well known in the art, the amplitude of an alternating current in an RLC circuit can be controlled by varying the frequency of the current. A given RLC circuit will have a resonant frequency, at which the current amplitude will reach a maximum value. From an efficiency standpoint, to operate an induction furnace at its resonant frequency will maximize the energy transferred into the melt charge. However, to operate an induction furnace at its resonant frequency is impractical, as will be explained in detail below.
FIG. 1 is a block diagram of a typical induction furnace. External power is provided from a commercial source, and is usually in the form of 60 Hz AC from the power mains. The 60 Hz AC is rectified to provide high voltage DC. The DC is fed into an inverter 10, which usually utilizes silicon controlled rectifiers (SCRs) to "chop" the DC voltage into a square wave shape. The frequency of the "chopping" is determined by the frequency of the SCR firing. The speed at which the SCRs are fired thus controls the frequency of the resulting square wave. The square wave is then fed into the RLC circuit, in which the melt charge and induction coil may be regarded as a core disposed within an inductor L. As is well known, when alternating voltage is fed into an RLC circuit, a current having a sine-wave shape flows in the RLC circuit. The frequency of the voltage square wave and the resulting current sine-wave is directly controlled by the frequency of the SCR firing.
FIG. 2 shows a typical type of inverter (such as the inverter shown in FIG. 1), a "full-bridge" inverter 10, connected between a DC source 12 and the RLC circuit 14. (The n.sup.2 R term at 14 represents the equivalent resistance of the RLC circuit, taking into account the number of turns n in the coil and the resistance R of the melt.) The full-bridge inverter 10 comprises four diodes 16 as shown, and four SCRs which operate in pairs 18a, 18b, and 20a, 20b, respectively. The SCRs operate as switches which complete a circuit when they are "fired" (i.e., rendered conductive) by an external control signal. In a fullbridge inverter, the SCRs 18a, 18b and 20a, 20b are turned on and off alternately in pairs at the desired frequency for the square wave. The arrows in FIG. 2 show the direction of current from the DC source 12 when SCRs 18a, 18b are fired and SCRs 20a, 20b are left open (i.e, nonconductive). SCRs 18a, 18bcomplete a circuit from which DC from source 12 flows through the RLC from left to right, as can be seen by the arrows. If, alternatively, SCRs 18a and 18b are in a non-conductive state, and SCRs 20a and 20b are fired, current will flow in the opposite direction through RLC 14, from right to left. As those skilled in the art will understand, an SCR, once fired, will conduct electric current as long as this current flows from the SCR's anode terminal to the cathode terminal. Should the current change direction, the SCR will block conduction and after a short period, usually 30-70 .mu.sec, will turn off and again become non-conductive. This period is called "turn-off-time" or TOT, for short.
FIG. 3 shows a series of curves graphically describing the behavior of the current of FIG. 2 in the course of one and a half cycles of the inverter 10. With reference to curve 100, which describes the current associated with the inverter over time, and curve 110, which describes the power associated with the inverter over time, the action of inverter 10 can be summarized thus:
The above summary will now be explained in detail. When DC is input into an RLC circuit, the circuit will "ring", and oscillations of voltage and current will result. The frequency of these oscillations depends on the specific values of the RLC components, including the properties of the melt charge inside the inductor. When SCR pair 18a, 18b is fired, current flows through the RLC circuit and the inverter in the direction of the arrows (FIG. 2). Current will gradually build up to its maximum value and then subside to zero, as illustrated in curve 100 of FIG. 3. The total energy passed from the DC source to the melt charge during the interval t.sub.0 -t.sub.1, half a period for the oscillation of the RLC circuit, is: ##EQU1## where v and i are voltage and current in the RLC circuit, respectively.
During this half-cycle, charge accumulates on the capacitor. At time t.sub.1, the voltage on the capacitor is larger than the DC voltage and the capacitor begins to discharge, reversing the direction of the current along the path given by the arrows in FIG. 2. This reversal of current will cause SCRs 18a, 18b to turn off. After the turn-off-time (TOT) of SCRs 18a, 18b, this pair of SCRs will become non-conductive (although current can still return to the DC source through the diodes 16). For the period between t.sub.1, when the capacitor begins to discharge, and t.sub.2, when the other set of SCRs 20a, 20b is fired, the extra energy stored in the capacitor is returned to the DC source. The energy returned to the DC source between t.sub.1 and t.sub.2 is given by: ##EQU2## This reversal of current is illustrated in curve 100 of FIG. 3 as the negative portion of the curve between t.sub.1 and t.sub.2, encompassing shaded area 101a.
Normally, in a full-bridge inverter and many other types of inverter, the other pair of SCRs will be fired at some time after the turn-off-time of one pair of SCRs. When the other pair of SCRs 20a, 20b are fired, the DC from the source 12 flows through the RLC from right to left in FIG. 2, and the capacitor, begins to charge to the opposite polarity. Between points t.sub.2 and t.sub.3 in curve 100 in FIG. 2, the voltage and current relative to the DC source have the same polarity and therefore the energy transferred to the load is positive: ##EQU3##
In summary, energy is passed from the DC source to the metal charge (via the coil) when the voltage and current have the same polarity. This condition exists, in curve 100, between t.sub.0 and t.sub.1 and between t.sub.2 and t.sub.3. During the period t.sub.1 to t.sub.2, and between t.sub.3 and t.sub.4, energy is not being passed to the coil but is being returned to the DC source. These periods of negative energy are shown as shaded areas 101a and 101b in curve 100 and 111a and 111b in curve 110. Over the period T of an operating cycle (from t.sub.0 to t.sub.4), the power produced by the inverter can be determined as: ##EQU4##
Assuming that the current is a sine wave and the voltage a square wave, as would be the case with such an inverter, the power passed from the inverter to the furnace will be equal to: ##EQU5## where: V--inverter voltage (=V.sub.DC for a full-bridge inverter);
The key to equation (5) is the relationship of the phase difference .phi. and the time interval t within each cycle in which energy is being returned to the DC source. From FIG. 3, it can be seen that for every cycle of inverter current (t.sub.0 to t.sub.4), there are two periods of equal duration in which power is returned to the source. These periods are the same as the periods between the zero crossing of the current and the zero crossing of the voltage in the inverter, which can be seen by a comparison of the zero crossings of curve 100 and curve 108. It is clear from equation (5) that, for .phi. between 0.degree. and 90.degree. , an increase in .phi. will cause a decrease in power. Thus, as .phi. increases, power passed to the furnace decreases. Maximum power transfer occurs when .phi.=0.
However, a dangerous condition exists in an RLC circuit at resonance, in which .omega..sub.1 equals .omega..sub.0. Resonance is the point of maximum power transfer, when there is zero phase shift between voltage and current in the inverter. Zero phase shift means, in effect, that one set of SCRs is being turned on at exactly the same instant the other set is being turned off. This would be no problem if SCRs behaved as idealized switches, which open instantly. However, there is a finite period of time, the turn-off time (TOT) during which an SCR is still conductive after being turned off. If the phase shift is less than the TOT of the SCRs, all of the SCRs will be conductive at the same time, thus causing a short across the DC source. Thus, in order to avoid shorting out the power supply, the phase shift between voltage and current must always be greater than the TOT of the SCRs. This amounts to the same thing as preventing the frequency of the DC chopping from approaching the resonant frequency of the RLC. In order to operate safely, the frequency of SCR firing must always be safely below the resonant frequency of the RLC.
The engineering problem posed by this requirement is that the resonant frequency of an induction furnace does not remain constant but may vary considerably in the course of use. The physical properties of the melt charge, which acts as the inductor core, have a direct and significant effect on the resonant frequency of the furnace. These significant physical properties include the temperature of the melt charge at any given point of the heating operation, the amount of metal in the furnace at any given time, and the specific composition of the alloy being heated. These properties will vary widely with every situation, and even within the course of a single use of the furnace. It is not uncommon in induction melting to add cold metal to the furnace while a previously added batch is still heating, thus changing the mass, temperature, and crystal structure of the core almost instantaneously, and thereby almost instantaneously changing the resonant frequency of the inductor.
Of course, the SCR firing frequency could be kept extremely low so that the phase shift will always be greater than TOT, even at resonance. This approach is unacceptable because the power supply would become extremely inefficient. Because it is crucial that the input frequency be less than the resonant frequency, and because the resonant frequency may change so suddenly, a control system to control SCR firing frequency in response to new physical conditions in the furnace is required so that phase shift may be minimized for high efficiency yet never less than TOT to avoid shorting the power supply.
It is also theoretically possible to calculate the resonant frequency of an induction furnace at any given instant, given the instantaneous temperature, the mass of the core, and physical properties of the core, and thereby change SCR firing frequency as required but as a practical matter these parameters are too difficult to measure, and are not suitable as inputs to a control system.
One common attempt at solution to this problem is varying the inverter frequency electronically, using voltage-controlled oscillators. The voltage-controlled oscillators generate pulses with a frequency proportional to a control voltage produced by a closed-loop circuit which measures the output power and compare it with a preset desired value. However, this method has a major drawback in that a frequency control system generally cannot adapt to sudden changes in electromagnetic properties of the furnace. If a cold charge is dropped into the melt, the system is likely to encounter the new resonant frequency before the frequency can change, and the inverter will crash. Special protection circuits to detect such a condition are cumbersome and do not work well.
In contrast, the present invention controls power delivered to the induction coil by varying the phase difference between current and voltage in the coil in response to the resonant frequency of the load. The present invention does not directly vary the frequency of the inverter AC voltage. Instead, the present invention monitors the zero-crossings of the current in the inductor, and adjusts the time delay before the SCR's are fired in such a way that the output power level is maintained and that there will always be at least a minimum phase shift .phi. between current and voltage. Although the frequency of the DC voltage may vary in the course of use of this method it is important to understand that the method merely reacts to the resonant frequency in the RLC load circuit under a variety of conditions.